Embodiments of the present invention relate to compositions and methods for diagnosing and treating Phenylketonuria (PKU).
By a mechanism not completely understood, PKU patients suffer from mental retardation, epilepsy, organ damage, unusual posture and, in cases of maternal PKU, severely compromised pregnancy. Classical PKU is an autosomal recessive disorder, caused by mutations in the PAH gene, located on chromosome 12. Mutations in both alleles of the gene result in remarkably high concentrations of phenylalanine2, 3. Excessive phenylalanine is partly metabolized into phenylketones by transamination, however in untreated patients millimolar concentrations of phenylalanine can accumulate in the plasma, cerebrospinal fluid (CSF) and brain tissue5, 6. Inclusion of PKU diagnosis to newborn screening programs, by semiquantitative methods such as the Guthrie test or modern analytical tools to measure the blood levels of phenylalanine, allows early diagnosis of affected patients. This permits treatment with phenylalanine-restricted diet before clinical symptoms appear. In adults who do not keep a strict diet there is a risk of late motor and cognitive decline3. In most of the previous studies phenylalanine is considered to be the main neurotoxin, although the precise mechanism underlying the neurologic affect still needs to be deciphered3.
In the past decade, the role of peptide and protein aggregation in many pathological disorders was revealed. Specific attention was drawn to the formation of ordered amyloid fibrils. It was clearly demonstrated that amyloid fibrils or their early intermediates are associated with a diverse group of diseases of unrelated origin, including Alzheimer's disease, Type II diabetes, and prion disorders. Despite their formation by a diverse and structurally unrelated group of proteins, all amyloid fibrils share similar biophysical and structural properties4, 7-12.
A very intriguing point is the fact that very short peptide fragments, as short as penta- and tetra-peptides, can form typical amyloid fibrils that share the same biophysical and structural properties of the assemblies formed by much larger polypeptides13, 14. Furthermore, it was demonstrated that a diphenylalanine peptide spontaneously forms well ordered nano-tubular assemblies by itself, with some amyloid-like structural signatures18. This short peptide represents the core recognition motif within the β-amyloid polypeptide, which forms amyloid plaques in the case of Alzheimer's disease. The two phenylalanine residues (Phe19, Phe20), in the β-amyloid polypeptide, are suggested to mediate the intermolecular interaction between polypeptide chains, further substantiated by being a key component of peptide-based inhibitors of β-amyloid fibril formation15-17.
A variety of structural and biophysical studies indicate the role of interactions between aromatic residues in amyloidogenic process acceleration and amyloidal structure stabilization. While aromatic interactions are not crucial for the process of amyloid formation they can significantly accelerate the process, affect the morphology of the assemblies, and reduce the minimal association concentrations13, 19-22.
To date, PKU has not been associated with amyloid-like fibrillar load.